Lead researcher Professor Richard Telford said the results of the Lifestyle of our Kids study at Canberra Hospital showed lean children did not necessarily eat fewer kilojoules than overweight children, but they were much more physically active.

''General community strategies involving dietary intake and physical activity to combat childhood obesity may benefit by making physical activity the foremost focus of attention,'' he said.

In the four-year study of 734 otherwise healthy children aged between eight and 12, researchers used pedometers to measure physical exercise and nutritionists measured dietary intake.

Professor Telford said the study found lean boys actually ate more kilojules than their overweight counterparts, but they made up for it by doing a lot more exercise. There was hope for the parents of overweight children though, with evidence that children who exercised more during the four years became slimmer and, those who exercised less fat.

Deakin mother-of-three Vanessa Stephen was concerned by the levels of obesity among children. ''It scares me,'' she said. ''I also feel sympathy for kids that are in that position, because it's got to be really tough not only physically but mentally.''

Her three children Mac, 8, Emily, 7, and Will, 4, have weekly swimming lessons - Emily has a dance class every week, Will rugby skills training and Mac plays rugby, cricket and has more swimming classes through his school. They also ride their bikes and play in their front yard.

Mrs Stephen considered her children were naturally reasonably active, and she and husband Rob were managing to juggle getting them to and from their various sports. She conceded that fitting it all in might become more challenging as the children got older.

''If you don't make the effort, they don't do it,'' she said.